This invention is particularly concerned with thermoforming operations in which a web or sheet of thermoplastic material is first softened by heat and then formed or shaped while in this stretchable plastic state by being drawn or pressed against the contours of a mold or die under the influence of a fluid pressure differential. The former may be exemplified by the application of pressure within a pressure box produced by compressed air or the like, to press the softened material against the contours of a male mold or core pin. Alternately, a vacuum may be applied from within a female mold while allowing atmospheric pressure to shape the heat softened thermoplastic material against the surface of such female mold. For many purposes, a combination of these two techniques is preferred with the softened sheet material being subjected to vacuum on the face contacting the mold surfaces and to a substantial positive pressure on its other face as such combination provides greater versatility and better control in the forming operation.
The apparatus for effecting thermoforming operations, in general, is comprised of a press having upper and lower platens on which are mounted a male mold assembly and female mold or pressure box assembly. In this regard, the male mold is generally mounted on the lower platen, however, this is normally a matter of choice. Prior to forming of the sheet of heated thermoplastic material as hereinabove discussed, a clamping assembly normally constituting a part of the male mold assembly is first caused to trap the sheet of thermoplastic material about the entrance into the female mold of the pressure box assembly including a cooperating clamping grid. Thereafter, the male mold assembly is caused to pass through the plane of the thermoplastic material and eventually form the hollow article, as hereinabove discussed. The relative movement of the male and female molds is effected by hydraulic cylinder assemblies and associated equipment moving the respective platens on which such assemblies are mounted whereas movement of the clamping assembly is effected by an associated cylinder assembly of a mold. Generally, tonnage clamping pressures are required necessitating high pressure operations.
Such hydraulic cylinder assemblies are large even for small thermoforming apparatus and have high energy requirements. In this application the term fluid cylinder assembly is to be interpreted as the combination of a cylinder, piston and rod which is operated by a fluid including air, gas or a liquid, such as a hydraulic fluid, whereas a gaseous cylinder assembly is a similar combination of elements motivated by a gas.
In copending application U.S. Ser. No. 717,636, assigned to the same assignee as the present invention, there is disclosed a novel process for thermoforming hollow plastic articles wherein the relative movement of the male mold and female mold are effected by fluid, preferably gas cylinder assemblies rather than hydraulic cylinder assemblies which move the male mold assembly and female mold assembly including the male mold and female molds respectively.
In all such thermoforming operations, the formed article has a tendency to become distorted or disformed prior to removal from the supporting sheet of thermoplastic material, particularly large L/D hollow articles or bottles as a result of non-support of the thus formed article between forming and removal. Present technology limits L/D ratio to less than about 7:1.